🚗

Tire Size Comparison Calculator

Compare two tire sizes side by side — overall diameter, circumference, sidewall height and the speedometer error from switching.

Old tire width

Old aspect ratio

Old rim diameter

New tire width

New aspect ratio

New rim diameter

Loading calculator…

How it works

Sidewall height = width × aspect ÷ 100 · Overall diameter = rim × 25.4 + 2 × sidewall (mm) · Diameter difference = (new − old) ÷ old · Actual speed = indicated × new diameter ÷ old diameter

A tire’s overall diameter is the rim diameter plus two sidewalls, and the sidewall height is the section width times the aspect ratio. When you change size, keeping the overall diameter within about 3% of stock preserves speedometer accuracy, gearing feel and clearance. A larger diameter makes the speedometer read low (you are actually going faster than it shows) and effectively taller-gears the car; a smaller diameter does the opposite.

Worked example

205/55R16 has a diameter of 16 × 25.4 + 2 × (205 × 0.55) ≈ 632 mm. 225/45R17 is 17 × 25.4 + 2 × (225 × 0.45) ≈ 634 mm — just 0.4% larger. At an indicated 60 mph you would truly be doing about 60.2 mph, so the speedometer stays essentially accurate.

Frequently asked questions

How much can I change tire size safely?

A common guideline keeps the overall diameter within about 3% of the original, and the width within what the wheel and fenders allow. Larger deviations affect speedometer accuracy, ABS/traction calibration, clearance and load rating — check fitment and load index before switching.

Will bigger tires make my speedometer wrong?

Yes. A taller tire covers more ground per turn, so the car goes faster than the speedometer indicates until it is recalibrated. This tool shows the true speed at an indicated 60 mph so you can judge the error.

What does the aspect ratio mean?

The middle number of a tire size (the 55 in 205/55R16) is the sidewall height as a percentage of the tread width. A lower aspect ratio means a shorter, stiffer sidewall — sharper handling but a firmer ride and less protection from potholes.

Related calculators

Tire Size Calculator

Convert a tire spec like 225/45R17 into overall diameter, sidewall height, circumference and revolutions per mile.

Speed from RPM and Gearing Calculator

Calculate vehicle speed from engine RPM, gear ratio, final drive and tire diameter — or find the RPM at a given cruise speed.

Final Drive Ratio Calculator

Find your differential final drive ratio from ring and pinion teeth, and the overall ratio in any gear.

Gear Ratio Calculator

Find the axle gear ratio from ring and pinion teeth, plus the engine RPM at a given speed.

Wheel Offset and Backspacing Calculator

Convert wheel offset (ET) to backspacing and front spacing or back again, and compare a new wheel with your current one: how far the outer lip pokes out, how much inner clearance you lose, track width change, wheel spacers and tire width.

Educational estimate — real-world figures vary with load, temperature, traction, tuning and measurement method.